For example, JP2006-511078A (hereinafter, referred to as “document 1”) discloses a supply assembly, which is configured to supply electric power to an LED (light emitting diode) module. This supply assembly includes: a series circuit of a diode and a control switch; and a controller. The series circuit is connected across both ends of a DC power supply. The controller is configured to output a dual PWM (Pulse Width Modulation) signal to the control switch. In addition, a series circuit of an inductor and the LED module is connected across both ends of the diode.
The controller includes a pulse width modulator, to which a current reference signal from a current source, a detection current and a high frequency saw-tooth-wave signal are input. The pulse width modulator is configured to output a high frequency PWM signal to one input part of an AND gate. To the other input part of the AND gate, a low frequency PWM signal is input. The AND gate is configured to output the dual PWM signal that is obtained by combination of the high frequency PWM signal and the low frequency PWM signal. The dual PWM signal is input to a gate of the control switch via an amplifier.
In this supply assembly, it is possible to change the intensity of light to be output from the LED module by changing a low frequency component of the dual PWM signal to be output from the controller to the control switch.
Incidentally, generally, a luminance of an LED is higher than that of an incandescent lamp or a fluorescent lamp. Therefore, if the LED is lighted by the same luminous flux as the incandescent lamp or the fluorescent lamp, a user may sense that the LED is brighter than the incandescent lamp or the fluorescent lamp. For this reason, it is required to reduce the luminous flux by suppressing a drive current for the LED, in order to make the apparent brightness of the LED agree with the brightness of the incandescent lamp or the fluorescent lamp.
In a case of an illumination apparatus including one or more LEDs in which a rated current is 350 [mA] for example, it is required to set a lower limit of the current to be equal to or less than several microamperes to several milliamperes. However, it is hard to perform minute current control as described above, in a so-called burst light method of controlling light so that a load current intermittently flows through the LED module, as the supply assembly in the above-mentioned document 1.
In addition, when fade-in control or fade-out control is performed to the LED, it is required to control the drive current for the LED over a wide range of a minute current value to a rated current value in order to smoothly change the brightness of the LED.
When the rated current for the LED is 350 [mA] for example, it is required to adjust the drive current in a range of 100 [μA] to 350 [mA], and accordingly, the drive current should be controlled by the resolution of about 1/1000. For this reason, it is hard to smoothly change the brightness of the LED in the case of the supply assembly in the above-mentioned document 1.